This application claims the priority of German Application No. 198 22 855.4, filed May 22, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method and a system for monitoring and/or documenting known laser machining operations particularly via a pulsed laser, wherein a detector at least indirectly, and partially, detects the distribution of the energy flux density of the laser beam.
To measure a laser beam requires measuring a large number of time-related and site-related beaming data. The measurability as well as the informational value of these data depends largely on the beam source which is used. With respect to the laser, a differentiation must be made between pulsed operation (p) and the continuous-wave operation (cw). Furthermore, in the case of pulsed operation, the pulse duration tp and the pulse period T or the pulse frequency fp=1/T must be considered to be largely independent values. In contrast to pulses, which are generated by acousto-optical modulators (Q-switch), in the case of lamp-pulsed systems, the above-mentioned values are mutually dependent to a high degree.
The distribution of the energy field density or intensity in the plane perpendicular to the propagation direction of the laser beam I (x, y) is an important measurable variable during the lasering of material. With respect to its lateral distribution, this measurable variable is responsible for the symmetry of the ablated workpiece volume (particularly important during the laser drilling and removal process). The energy flux density, which in a first approximation is responsible for the removed material volume, can be calculated as the product of the intensity and the pulse duration.
It should be noted that measuring such an intensity distribution should best take place in focus or close to focus (Ifocus(X, y)). There are two important reasons for this: (1) possible additional disturbance variables (lens, deviation mirror, thermal refractive effects, phase front deformations . . . ) are included in the measurement; and (2) this selection of the measuring site also makes it possible to directly measure geometrical beaming data (focus diameter df, beaming quality, beam position, . . . ).
Currently, measuring instruments are available on the market for only a few measuring tasks (CO2-laser, cw-operation). Particularly in the case of fast and short-pulsed lasers, virtually no adequate measuring instrument is available. Mainly within the wavelength range of 1,064 nm and 532 nm (Nd:YAG), CCD-cameras are used as site-resolving sensors. For measurements in focus or close to focus, systems are used which are based on the principle of a rotating needle or rapidly moving screens. However, the above-described measurable variables can be sensed by means of such systems only in a few exceptions. In particular, the transversal intensity distribution of Nd:YAG lasers in focus or close to focus: Ifocus (x, y), cannot be measured by the offered instruments.
The only systems which can manage the high power densities of the lasering focus (rotating needle or similar devices) have, among other things, an insufficient site resolution for the wavelength of the Nd:YAG laser. In addition, this method is basically unsuitable for pulsed lasers because the measurement of an intensity profile is composed of many partial measurements of different pulses.
For measuring laser pulse groups (hereinafter a laser pulse group is defined as a number of several directly successive laser pulses) and of individual laser pulses, CCD cameras or similar site-resolving sensors are particularly suitable. This is because these can record an intensity distribution in a measuring operation. However, these systems, which are based on CCD-sensors, so far have no suitable triggering possibility for recording individual laser pulses. In addition, with the exception of very costly high-speed cameras, they are unable to record successive laser pulses. CCD cameras, as they are partly used in beam diagnostic modules, operate at 25-60 Hz. Based on normal frequencies of laser pulses (percussion drilling 10 Hz -10 kHz), an integration takes place in the extreme case by way of a laser pulse group of approximately 150 to 300 individual laser pulses. In addition, none of the CCD diagnostic systems can be used in focus or at least behind the lasering lens system.
It is an object of the present invention to develop a method and a system by which, also in the case of high pulse rates of the laser beam, an improved diagnosis and documentation of a machining operation carried out by means of a laser is permitted. The object is achieved by a method for monitoring and/or documenting a machining operation carried out by a laser, particularly a pulsed laser beam, characterized in that, in a combined manner, (1) the laser beam is aimed at a machining point; (2) at least indirectly a measuring beam (2.1.), which is preferably constantly correlated with the laser beam, is coupled out of the machining laser beam; (3) the measuring beam is deviated in a defined manner at different times and in a manner which is defined and locally mutually separate; and (4) the distribution of the energy flux density of the pulses or pulse groups of the measuring beam, which are locally separately deviated in a defined manner, is measured. A system to implement the method is characterized in that, in a combined manner, (1) the system has an extraction device for the at least partial intensity-side extraction of a measuring beam preferably constantly correlated with the laser beam; (2) the extraction device is arranged within the beam path of the laser beam; (3) the system has a deviating device; (4) the deviating device is arranged in the beam path of the measuring beam; (5) the deviating device has at least one deflecting device which, as a function of the time, deflects the measuring beam into defined directions in space and divides it into various partial beams, for example, a mirror which is controlled by a rotary step motor; and (6) the detector is arranged in the area of partial beams of the measuring beam which are deflected by the deflecting device.
In addition to achieving the above object, the present invention also has the following advantages:
(1) It characterizes the laser beam sources as well as time-related beaming characteristics;
(2) By means of the improved time-resolved measurement of a pulsed laser beam, informationally valuable data can be obtained for the process simulation (laser drilling/laser removal);
(3) A process monitoring (on-line) is possible for quality control during manufacturing operations with pulsed beam sources, particularly in the case of laser ablation, laser drilling and/or laser welding processes; and
(4) Systematic and/or statistical machining defects can be diagnosed and analyzed more precisely because, by using an informative beam diagnosis, it can clearly be decided whether the defects originate from fluctuations/defects of the material or from fluctuations asymmetries of the beaming source.
With respect to all fields of laser machining, it is generally applicable that the method according to the invention and the system according to the invention can be used in a simple manner, preferably in manufacturing and production. In particular, the beam diagnostic possibilities according to the invention are easy to operate and can easily be adapted. It is also a special advantage that, during a measurement in or behind the machining focus, no intervention is required in the beam guiding of the machining equipment.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.